Understanding kleptoplasty in Sacoglossa (Heterobranchia, Gastropoda) - a cellular histochemical and genomic approach

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Sacoglossa is an enigmatic heterobranch group, well known for some members ability to incorporate chloroplasts (cps) from their algal food and to survive weeks and months without additionally feeding. Recent transcriptome analyses reject old hypotheses, which suggested a horizontal gene transfer between the algal nuclear genome and the slugs nuclear genome as the basis for this interaction. Thus, the retention of cps cannot be explained by genetic support via gene transfer. New hypotheses must therefore shift to interactions and specific properties of cps and slugs. Our knowledge on Sacoglossa has increased considerably in the last years, including information on phylogenetic relationships, on food specificity, slug behaviour and photosynthetic capability. This information explains several factors that enhance cps maintenance and slugs survival, however only few studies have dealt with the properties on cellular level. Former results (including our own) indicate a specific survival of only few chloroplast types, a time specific survival of cps dependant on starvation and food availability, as well as slugs ontogenetic phase. This project aims to clarify the cp incorporation processon cellular level and explore the role of the cps in the slugs digestive gland cells, especially concerning their contribution as a long-term energetic storage device with the ability to increase nutritional value over a certain period of time. This investigation will utilize ultrastructural, immunohistochemical and confocal laser microscopical methods to determine the path each incorporated cp takes once ingested. Quantitative methods (morphometry, quantitative real time PCR (qPCR) and biochemical analyses) will help to elucidate the contribution of the cps as nutrional depots. Study of gene expression will help to understand physiological and genetic properties of the slugs during various starvation phases. We focus on sister-taxa, Elysia timida and E. cornigera, both of which feed on Acetabularia. E. timida can retain functional cps for months whereas E. cornigera can only retain cps for a few weeks